This invention relates to a saw chain such as mounted on a chain saw or tree harvester and used for sawing and bucking trees, and more particularly to a saw chain that can be sharpened automatically.
Automatic sharpening of saw chains for hand held chain saws was developed in the 1960""s. These chains are typically referred to as top sharpening chains in that they are designed to be sharpened from the top side of the cutting tooth rather than at the bottom side as in conventional saw chains. Top sharpening can be accomplished with a sharpening stone that is simply moved from an overlying position into contact with the protruding cutting edge with the chain mounted to the bar and while the chain is traveling around the bar. A stone mounted in the chain saw housing is lowered into contact with the cutting edge simply by pressing a button (automatic sharpening). An example of such chain is illustrated in U.S. Pat. No. 4,625,610.
There are two principal problems associated with top sharpening saw chain. There is a concern for potential kick back as the saw chain passes around the nose of the bar (a safety concern) due to the inherent design of the chain. Also, the chain is difficult to produce, most notably in the application of chrome to the cutting tooth. Chrome is desirable for maintaining sharpness and is applied to the side of the cutting tooth opposite the side that is filed for resharpening. In a top sharpening chain, this requires that chrome be applied to the underside of the cutting tooth. As can be observed in the ""610 patent, this side of the tooth is located in a tightly spaced cavity that is not easily accessible for applying the chrome and the chrome is either not applied adequately or the cost of doing so is excessive.
In the field of harvesting trees, the operation of cutting trees has substantially shifted from manual cutting, i.e., loggers wielding hand held saws, to machine cutting, where a chain saw is mounted on an articulated boom carried by a mobile machine, the saw being manipulated from the safety of an operator""s cab. Such mobile machines are often referred to as tree harvesters.
In tree harvester cutting, the concern for kick back is minimal. However chain sharpness is a substantial concern. A tree that is being cut and particularly in bucking the tree length into, e.g., eight foot lengths, is prone to splitting. Such splitting is alleviated by the increased speed in cutting through the tree and vice versa, splitting increases as the cutting speed is reduced. A sharp versus dull chain cuts faster and thus reduces splitting, a major concern for trees being cut for lumber production.
Tree harvesters are very expensive to purchase and operate and can be justified only because of increased cutting efficiency. To interrupt operation of a tree harvester so as to replace a dull chain with a sharp chain is a balance of efficiencies. A sharp chain cuts through a log in less time and reduces splitting occurrences. Also, a sharp chain requires less force while cutting and increases the wear life of the chain and bar, i.e., at the surfaces where the chain slides along the bar edge. However, these advantages have to be weighed against the undesired down time to replace the dull chain with a sharper chain. A further consideration is operator resistance. Particularly in bad weather conditions, the operator of the machine is disinclined to leave the comfort of the cab to wrestle with chain removal and replacement. Tree harvesting can accordingly be benefitted by automatic sharpening which eliminates the need for replacing the chain in order to maintain cutting chain sharpness.
For the purpose of explaining this invention, the cutting link is considered to have a chassis and a cutting portion that is extended upwardly or outwardly from the chain (the chain being a continuous loop and the cutting portion extending away from the loop center) which cutting portion is formed into a cutting tooth.
Typically a depth gauge also extends upwardly from the chassis and leads the cutting tooth so as to limit the depth of cut to be taken by the cutting tooth.
As will be appreciated, prior top sharpening chains, e.g., as shown in the ""610 patent, provide a top sharpenable cutting edge by lateral folding of the cutting portion in a manner to create a top plate that is severely sloped downwardly front to back. By grinding the front of the top plate to provide a flat top surface substantially parallel to the rivets, a sharp leading edge is created between the thus created top surface and the under surface of the top plate. This is the cutting edge and it is resharpened by repeated grindings of the top surface. (Whereas the top surface is described as being flat, because it is ground as the chain travels in a curve, the top surface is slightly curved downwardly front to back and provides a desired relief that alleviates rubbing of the top surface along the kerf bottom, this being a known advantage to the industry.)
Forming the present top sharpening chain is schematically illustrated in FIG. 1A. It should be first noted that there is no lateral folding to produce a top plate. The cutting portion 10 of the cutting link 22, before forming, is shown in solid lines. The cutting portion 10 is not folded as in the ""610 patent and instead is simply bent as shown in the dash line position. The pertinent features of the cutting portion (solid line) includes a top edge surface 12, a side surface 14 and edge line 16 between surfaces 12 and 14. In the preformed condition, the cutting portion 10 has a leading edge surface 18 and edge line 20 between surfaces 18 and 12. The dash lines illustrate the forming of the top plate into a right hand cutter link. The forming operation produces bending of the cutting portion 10 toward one side, e.g., to the right as shown in dash line. As thus bent or curved, the edge line 16R in the area of the bend becomes the leading cutting edge. The edge 20R is shaped by grinding off the material that protrudes laterally beyond point 40 (the juncture of edge line 16R and edge line 20R) and as thus shaped forms the side cutting edge of the cutting link. In production, as presently contemplated, the leading edge 16R is also finally shaped in production by grinding. Edge surface 12 is sloped downwardly and rearwardly whereby flat grinding of the top of the leading portion of the cutting portion resharpens the cutting edges.
It is considered preferable to form the cutters of the present invention as the center or drive links of the saw chain as shown. The cutter as shown in dash lines in FIG. 1A is a right hand cutter link. A left hand cutter link is formed by forcing the front of the cutting portion 10 to the opposite side. The combined width of the left and right hand cutting edges 16R and 16L extend laterally the width of the chain as illustrated in FIG. 1B (the cutters shown in overlapping relation for illustration purposes). The cutting links of FIG. 1B are assembled by rivets 28 to side links 26.
Also illustrated in FIG. 1B (schematically) is a sharpening stone SS that is to be mounted on a chain saw housing, e.g., of a tree harvester. The sharpener SS is mounted for limited and controlled movement up and down as indicated by the arrows. As will be apparent, the sharpening stone resharpens the cutting edges 16 (16R and 16L) of the cutting links with controlled up and down movement of the sharpening stone. The sharpening stone additionally maintains the desired depth gauge height as will be described.
Whereas the above is believed to generally describe the invention, reference to the following detailed description and accompanying drawings will provide a more complete understanding and enablement for persons skilled in the art.